Railroads that have implemented Positive Train Control systems (e.g., ITCS, ETCS, I-ETMS etc) as overlays to existing signal systems, will ultimately desire to move towards a virtual signal system where no physical signals exist. Virtual signal systems can allow railroads to realize more capacity without having to physically install more signals and shorten existing signal blocks. Virtual signal systems also eliminate the maintenance associated with physical signals (for example replacing lamps, aligning signals, cleaning lenses, etc). In order to realize virtual signaling, the location of the front and rear of the train must be known as well as the integrity of the rail between trains.
WO2004/071839 A1 relates to a system for automatically detecting the presence of a train located within a detection or surveillance area of a railroad track associated with a railroad grade crossing. The system includes a transmitter unit that transmit a detection signal and a receiver that receives a detection signal.
WO 2007/134992 A1 relates to a device for detecting the occupied or free status of a section of track by means of a track current circuit including both rails of the section of track into which a transmitter introduces a transmitted signal which is supplied to at least one receiver via the track current circuit.